Parasites are masters of chemical warfare. To bypass their hosts’ many defense mechanisms, they have developed an amazing array of chemicals that can change and influence all kinds of body parts: Even brains. That’s right—the hub of a creature’s behavior, that biochemical miracle called the brain, can become a parasite’s ticket to life on easy street. From zombification to bad driving, here’s a look at microscopic mind-control at its ugliest.
George Romero might take credit for the Hollywood zombie, but parasitic jewel wasps have perfected the art of creating zombie cockroaches. The female wasp shoots venom directly into its cockroach victim’s brain, blocking a chemical called octopamine. In cockroaches and many other insects, octopamine stimulates energetic behavior like walking, flying, and paying attention to its surroundings. Without it, the cockroach is barely animate.
Dragging her zombie roach back to her lair, the jewel wasp lays an egg in the cockroach’s abdomen. It will live for seven or eight days … long enough for the egg to hatch and the wasp larvae to eat the still-passive cockroach from the inside out. In a world without refrigeration, a zombified dinner is one method for guaranteed good eats.
Strepsipterans are another insect with the ability to make zombies. The female strepsipteran is a parasite with no eyes and no legs that makes a home inside the body of other insects. Only its head will show, a faceless bulge sticking out of a convenient opening in the host’s exoskeleton. When it is time to mate, the strepsipteran releases pheromones that summon the (mobile and not parasitic) male of the species. The female strepsipteran waits patiently, emitting chemicals that force its host to stay motionless until the male arrives and the happy couple has completed their mating ritual. After witnessing this parasitic booty call, the poor host probably wishes the strepsipteran’s mind control came with a side dish of amnesia.
Finding good child-care takes money and hard-work—unless you’re a parasite willing to take control of your nanny’s brain. The ichneumonid wasp has found a way to make sure her baby stays safe and healthy by taking advantage of orb-weaving spiders. The wasp knocks out the spider with a sting, and during the few minutes the spider is paralyzed, the wasp glues an egg to the spider’s belly. The spider recovers and returns to normal spider life for a week or two, while the wasp egg hatches and the larval wasp grows strong drinking the spider’s body fluids.
After the baby wasp is old enough, it injects a psychotropical chemical into the spider that takes control of its brain. That night, the spider is compelled to weave a web, but the chemical disrupts it normal web-building patterns. Instead, the spider builds a strange, flat platform out of strong, unsticky silk. Before dawn, the larva molts, eats the exhausted spider, and uses the web platform as a base for building a cocoon where it can pupate into its adult wasp phase.
Another arachnid-turned-babysitter is the crab. A larval female sacculina barnacle abandons its free-swimming life to inject itself inside a crab’s exoskeleton. The sacculina spreads root-like fleshy tendrils throughout the crab’s body and creates chemicals that first sterilize the crab and then influence the crab’s secondary sex characteristics—any males infected with a sacculina parasite will begin to look like a female crab. As the sacculina matures, it grows a knob on the crab’s belly, in the same spot a normal female crab would develop a brood pouch.
The crab will carefully groom and clean the barnacle’s knob just as it would its own brood sac. And when the sacculina is ready to release her eggs (after punching a hole in the crab’s shell and sneaking in a mate), the crab will eject the barnacle’s eggs into the sea just like crab babies. The crab’s instincts, stimulated by the sacculina’s mind control, will tenderly stir the seawater to disperse the barnacle eggs around her, giving them the best start on their parasitic little lives.
These aren’t the only self-sacrificing nannies created by parasitic mind control. There are wasps whose larvae infect caterpillars. The larva then controls the mostly-dead caterpillar’s body as a bodyguard puppet. And, there’s a fungus which infects ants and forces them to latch onto plants until the fungus sends forth a fruiting body and disperses its spores. Fossilized leaves reveal the agonized bitemarks of infected ants going back at least forty-five million years. That’s a long legacy of stiffed babysitters.
It’s one thing to make your host act funny to score room and board or free childcare. But some parasites’ life-styles demand their hosts pay the ultimate price: Self-sacrifice. That can involve some pretty tricky biochemical puppeteering. The lancet fluke is a great example of a tiny puppet-master with remarkable control over its host.
Adult lancet flukes live inside the livers of grazing animals, and their eggs are distributed in animal feces. Most animals aren’t really excited about eating their own poop, so the lancet fluke depends on two other creatures to propel their babies into the next ungulate host. Snails, not the fussiest of eaters, will eat manure, gobbling up fluke eggs in their meal. The hatching eggs irritate the snails’ intestines, creating slimy cysts around a dozens of juvenile fluke. The snail vomits up the cyst, which somehow looks delicious to an ant. The flukes spread throughout the ant’s body, some moving into the primitive ant-brain, where they take control.
Ants cannot stay in direct sunlight because they will dry out and die, but a hidden ant is worthless to the lancet flukes. They drive the ant on, evening after evening, forcing it up stalks of grass and other tall plants. The ant will grip onto the grass and stay still all night, waiting around as long as it takes to be eaten by a grazing sheep or cow…whose liver, after all, is a lancet fluke’s dream home.
While the lancet fluke is good at its job, the hairworm is the master of murderous mind manipulation. Larval hairworms grow inside insects like grasshoppers and crickets. They begin as extremely tiny lifeforms, but by the time they’re ready to mate, their long hair-like bodies will fill a host’s entire body cavity. The hairworm creates proteins that stimulate the host to seek out water and jump in. As the host begins to drown, the hairworm shoots out the host’s rear end and begins looking for hairworm love.
If these self-destructive behaviors seem limited to the realm of lower creatures, don’t feel too comfortable. The tiny parasite Toxoplasma gondii reproduces only in the gut of cats, but it will live in all mammals. And toxoplasma creates chemicals with some interesting neurological effects. In rats, toxoplasma encourages more aggressive behavior, delayed reaction times, and an affinity for the smell of cat urine. All of these behaviors up the odds that infected rats will be eaten by cats, completing the toxoplasma’s life cycle.
But Toxoplasma gondii will live quite happily in humans. In fact, an estimated thirty to sixty percent of the world’s population is infected. The parasite will give most of its hosts something like a mild flu before settling into dormant cysts in nerve and muscle tissue. But toxoplasma’s biochemicals wreak some exciting havoc on their human hosts: Women with toxoplasma antibodies are almost twice as likely to have male children as women who are not exposed to the parasite. Behavioral changes are linked with infection as well—from low-level neuroses to aggressive behavior to full-blown schizophrenia. Strangest of all, people who are infected with toxoplasma are almost three times more likely to be involved in traffic accidents than uninfected drivers. The human brain, with all its complexity, has proven itself just as susceptible to microscopic mind control as any other creature’s.
As scientists learn more about parasitic life forms, their wide variety of powers and effects prove astonishing. With vampirism and zombie mind control a matter of course, these microscopic monsters are scarier than anything dreamed up by science fiction writers. After all, who knows how much of human history resulted from our free will … and how much came from the tiny puppet-masters steering our brains?
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